View full sizeBP PLC, The Associated PressThis image taken from video on Thursday at 3:57 shows that oil from the Macondo well had stopped pouring into the Gulf of Mexico.

The well was successfully "shut in" at 2:25 p.m., so that BP could begin a critical six- to 48-hour observation period that ultimately will determine whether the well is strong enough to continue holding the oil inside of it for up to a month until it can be permanently sealed with cement.

Echoing the sentiment of Gulf Coast residents, BP Vice President Kent Wells said it felt "very good" to no longer see oil spewing into the Gulf of Mexico, but he cautioned that scientists and engineers were still in the very early stages of testing the well and said results would not be known for several hours or possibly days.

"I'm very pleased that there's no oil going into the Gulf of Mexico. In fact, I'm very excited there's no oil going in the Gulf of Mexico," Wells said. "Where I'm holding back my emotion is we're just starting the test and I don't want to sort of create a false sense of excitement."

The integrity test involves measuring the pressure inside the well. If pressure rises and holds at 8,000 to 9,000 pounds per square inch, the well could remain closed. If it is lower than that level, however, it will be reopened and oil would again be sucked to vessels on the surface. The low pressure readings would indicate to scientists that oil is escaping through one or more fissures in the well.

"We will be monitoring the pressures carefully and every six hours we'll consult between our engineers and scientists and government scientists and make decisions to continue forward with the test," Wells said. "Or, if at any point we feel the test needs to be suspended we will do that."

BP officials did not report any results of the test Thursday night. But, BP Chief Operating Officer Doug Suttles told CNN there did not appear to be any leaks in the system or other immediate complications.

"We don't have any leaks. We don't have any oil coming out that we know of," Suttles said. "Hopefully we'll continue it for the next 48 hours, which puts us well into Saturday afternoon."

The condition of the well has been an unknown since the Deepwater Horizon rig exploded and sank April 20, killing 11 people and setting the stage for the protracted disaster. Concern about possible damage to the well was heightened after the failure of "top kill," an earlier attempt at stopping the oil flow by pumping heavy mud into the top of the well. Oil failed to overcome the flow of oil, raising the specter that it was escaping the well somehow.

Scientists estimate that the well produces 35,000 to 60,000 barrels of oil per day.

So far, controlling the leak subsea has meant simply collecting as much oil as possible and sucking it to vessels on the surface.

Before the shut-in attempt began last week, oil was being pulled from the well and onto the Discoverer Enterprise drill ship and Q4000 platform on the surface. The former collected oil for refinement, the latter flared it off on site. Together, the vessels managed to collect about 23,000 barrels of oil per day. The Discoverer Enterprise, which had been attached to an ill-fitting containment cap jammed on top of the well, was disconnected Saturday to make room for the shut-in system. A floating platform called the Helix Producer was also collecting oil at the site up until the shut-in process began.

The scientific team's worries were put to rest Wednesday and BP crews were given the green light to proceed with the well shut-in and integrity test. But the test was delayed again Wednesday evening after engineers discovered a leak in the "choke line" of a new blowout preventer placed on top of the well. The choke line was to be the last of five points closed in before the test began.

The leaking system was removed over night Wednesday and replaced Thursday morning with a back-up choke line that was already on site, Wells said.

If the well integrity test follows the best case scenario, oil would never leak from the Macondo well again and it would remain shut until a relief well could pump it with mud and seal it with cement, a procedure that would permanently shut the well down. BP crews have temporarily halted drilling of the relief well until the integrity test is complete.

The well has been drilled to about 17,840 feet. The plan is for it to intercept the Macondo well at about 18,000 feet.

If the integrity test results in the reopening of the well, BP would continue with an earlier plan that called for the continued collection of oil by surface ships until the relief well is complete.

As many as four ships, with the capacity to collect 80,000 barrels of oil and to disconnect quickly in the case of a hurricane, would be used in that process.

"Over the next several hours we will continue to collect data and work with the federal science team to analyze this information and perform additional seismic mapping runs in the hopes of gaining a better understanding on the condition of the well bore and options for temporary shut in of the well during a hurricane," Allen said. "It remains likely that we will return to the containment process using this new stacking cap connected to the risers to attempt to collect up to 80,000 barrels of oil per day until the relief well is completed."